Possible variants or subtypes of PPPD .................PPPD – persistent postural‐perceptual dizziness PPV – phobic postural vertigo SMD – space‐motion discomfort VV – visual vertigo CSD – chronic subjective dizziness

Commonalities among PPV, SMD, VV, CSD, and the syndromes that predated them formed the basis of the diagnostic criteria for PPPD. However, semiologic similarities, even those stretching over more than a century, do not necessarily mean that PPPD is a single entity. The current state of scientific knowledge left two important questions unanswered:  Is PPPD a single disorder with one principal pathophysiologic process or is it the common manifestation of multiple conditions that produce similar symptoms from different pathophysiologic mechanisms?  If PPPD is a single disorder, does it have clinically meaningful and validly distinguishable subtypes?